The present invention relates generally to radio communication systems and, more particularly, to techniques and structures for determining a channel on which a mobile station is camped.
In North America, digital communication and multiple access techniques such as TDMA are currently provided by a digital cellular radiotelephone system called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard TIA/EIA/IS-54; xe2x80x9cDual-Mode Mobile Station-Base Station Compatibility Standardxe2x80x9d, published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA), which is expressly incorporated herein by reference. Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), TIA/EIA/IS-54 is a dual-mode (analog and digital) standard, providing for analog compatibility together with digital communication capability. For example, the TIA/EIA/IS-54standard provides for both FDMA analog voice channels (AVC) and TDMA digital traffic channels (DTC). The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (KHz).
In a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user.
The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user""s digital traffic channel, which may be considered a logical channel assigned to the user. As described in more detail in TIA/EIA/IS-136, digital control channels (DCCHs) can also be provided for communicating control signals, and such a DCCH is a logical channel formed by a succession of usually non-consecutive time slots on the radio carrier.
The systems specified by the TIA/EIA/IS-54 and TIA/EIA/IS-136 (now ANSI136) standards are circuit-switched technology, which is a type of xe2x80x9cconnection-orientedxe2x80x9d communication that establishes a physical call connection and maintains that connection for as long as the communicating end-systems have data to exchange. The direct connection of a circuit switch serves as an open pipeline, permitting the end-systems to use the circuit for whatever they deem appropriate. While circuit-switched data communication may be well suited to constant-bandwidth applications, it is relatively inefficient for low-bandwidth and xe2x80x9cburstyxe2x80x9d applications.
Packet-switched technology, which may be connection-oriented (e.g., X0.25) or xe2x80x9cconnectionlessxe2x80x9d (e.g., the Internet Protocol, xe2x80x9cIPxe2x80x9d), does not require the set-up and tear-down of a physical connection, which is in marked contrast to circuit-switched technology. This reduces the data latency and increases the efficiency of a channel in handling relatively short, bursty, or interactive transactions. A connectionless packet-switched network distributes the routing functions to multiple routing sites, thereby avoiding possible traffic bottlenecks that could occur when using a central switching hub. Data is xe2x80x9cpacketizedxe2x80x9d with the appropriate end-system addressing and then transmitted in independent units along the data path. Intermediate systems, sometimes called xe2x80x9croutersxe2x80x9d, stationed between the communicating end-systems make decisions about the most appropriate route to take on a per packet basis. Routing decisions are based on a number of characteristics, including: least-cost route or cost metric; capacity of the link; number of packets waiting for transmission; security requirements for the link; and intermediate system (node) operational status.
Packet transmission along a route that takes into consideration path metrics, as opposed to a single circuit set up, offers application and communications flexibility. It is also how most standard local area networks (LANs) and wide area networks (WANs) have evolved in the corporate environment. Packet switching is appropriate for data communications because many of the applications and devices used, such as keyboard terminals, are interactive and transmit data in bursts. Instead of a channel being idle while a user inputs more data into the terminal or pauses to think about a problem, packet switching interleaves multiple transmissions from several terminals onto the channel.
Packet data provides more network robustness due to path independence and the routers"" ability to select alternative paths in the event of network node failure. Packet switching, therefore, allows for more efficient use of the network lines. Packet technology offers the option of billing the end user based on amount of data transmitted instead of connection time. If the end user""s application has been designed to make efficient use of the air link, then the number of packets transmitted will be minimal. If each individual user""s traffic is held to a minimum, then the service provider has effectively increased network capacity.
Packet networks are usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the TCP/IP protocol stack. These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks. The Internet is today""s most obvious example of such a standards-based network pursuit of this goal.
In a mobile packet data system, having a reservation-based medium access control (MAC) protocol over the airlink using acknowledged transmission, both low data latency and effective airlink bandwidth utilization are difficult to meet. One important technique used to address latency and efficient bandwidth utilization is load spreading, i.e., dynamically spreading the packets to be transmitted over various transmission resources. With the new standards currently being settled for integrated voice and packet data services in ANSI-136 systems, techniques will be established for spreading the traffic load not only within packet data channels on a single carrier, but also between packet data channels on different carriers.
For example, in systems including multiple packet data control channels (PCCH), on one or plural carriers, each mobile station (MS), through the use of a default hashing algorithm, chooses which PCCH it shall camp upon. More specifically, an inactive (idle) MS, performing a cell re-selection to a new cell, reads a beacon PCCH. On the beacon PCCH, information regarding PCCHs supported in the cell is provided. By using a default hashing algorithm the MS will determine which PCCH it shall use for its camping. The default hashing algorithm may, for example, determine the PCCH based on the least significant bit of the mobile station""s identification number. As a result, the mobile tunes to a selected PCCH and reads its paging channels.
Using this approach, the mobile stations in a cell are evenly distributed over the PCCHs supported by the cell. However, the generated traffic on each PCCH will vary depending on, for example, whether many of the MSs assigned thereto are active in a data transaction or whether many of the MSs assigned thereto are in a sleep mode and, therefore, reading only the paging slots.
In order to spread the traffic load as it changes due to changes in MS activity, the base station (BS) may use Channel Reassignment messages in order to force a mobile station to rehash to another PCCH or to order the mobile to move to a packet traffic channel (PTCH). If a mobile station is directed to tune to a PTCH, the mobile station will utilize the PTCH for the duration of the transaction and then return to the original PCCH and start a battery preservative procedure. If a mobile station is directed to tune to a PCCH on a different carrier, the mobile station utilizes the new PCCH for the transaction and optionally stays on the new PCCH after completion of the transaction.
In all of the above situations, the BS is required to remember which mobile stations have been ordered to rehash to another PCCH. As a result, rehashing lists in BSs are commonly quite long. Since inactive MSs are not required to send notifications to the network when entering a new cell (if within the same routing area), the BS ordered to page an MS has no way of knowing whether the MS is currently on a PCCH determined by the default hashing method or a PTCH/PCCH determined by a channel reassignment message.
Consider the following example which serves to better illustrate the problem confronting a base station seeking to page a mobile station which is assigned to a channel as described above. Suppose that a mobile station within a first cell, having channels A and B, uses its default-hashing algorithm to determine the channel to camp on. Suppose further that the default-hashing algorithm determines that the mobile station should camp on channel A. At some later time, the base station within the first cell transmits a Channel Reassignment message to the mobile station telling the mobile station to switch to channel B, due to, for example, the amount of traffic on channel A. If the mobile station then moves to another cell and subsequently moves back to the first cell, it will again use its default-hashing algorithm to choose channel A. When the base station wants to page the mobile station, it does not know whether the mobile station is camped on channel A (because it moved to another cell and then back to the original cell) or channel B (because the mobile stayed within the cell). In order to avoid this problem, the base station could page the mobile station on both channels, but this would increase paging traffic in the cell. Since bandwidth is limited, this is an undesirable solution.
Accordingly, it would be desirable to provide techniques and systems whereby a base station can more readily determine a channel on which to page a mobile station in a packet data communication system.
The present invention seeks to overcome the above-identified deficiencies by providing a rehash timer in both the mobile station and the base station in order to synchronize the channel selection process and to avoid the need to store long rehashing lists in the base station.
According to an exemplary embodiment of the present invention, a mobile station, which is camping on a first channel, receives a command from a particular base station to rehash to a second channel. The mobile station rehashes to the new channel, sends a confirmation message indicating the new channel to which it is listening and starts its rehash timer. Until the rehash timer expires, the mobile station will continue to use the second channel for communicating with that base station, even if it leaves the cell and subsequently returns. After the mobile""s rehash timer expires, if it needs to listen to that particular base station again, it will use the default hashing algorithm again to determine the appropriate (e.g., first) channel.
From the base station""s perspective, according to exemplary embodiments of the present invention, the base station receives the confirmation message from the mobile station which identifies the second (rehashed) channel to which the mobile station is now listening. At this time, the base station will store the new channel in a rehash list for later reference (e.g., if it needs to page the mobile station) and start its own rehash timer, which timer is associated with that particular mobile station. Until the base station""s rehash timer expires, the base station will page that particular mobile station using the channel stored in the rehash list. After the timer expires, the base station will use the default hashing algorithm to identify the appropriate (default) channel on which to communicate with the camping mobile station.